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The need for color management
Color reproduction is the primary task of printing, where color is at the heart of print life and quality control. However, for a long time, the colors between manuscripts, screen displays, proofing, and printing have often failed to reach agreement, causing unnecessary disputes.
There are many variable factors in the production copying process of color box, and the graphic and text need to go through many stages such as scanning, processing, color separation, printing, etc. It is not easy to make the original document be accurately restored. At each stage, the color information will be expressed in accordance with the color rendering characteristics of the equipment currently used, so different scanning and display devices will perform differently on the same original document. In addition, the RGB colored display screen and C, M, The color expression capabilities of Y, K or C, M, Y, K, LC, and LM color printing inks are different: In addition, there is a lack of consistency in the conversion data of different application software, and printed materials obtained using different papers and inks are also Different. Different types of devices, even devices of the same type and different types, have different capabilities for expressing colors.
Therefore, it is necessary to control the color reproduction on various devices and media in the production system by adopting the color management technology so as to ensure that the final replica can reproduce the original color better.
The concept of color management
Color management is the correct interpretation and processing of color information, that is, managing people's perception of color. The essence is the process of converting the color data of an image from one color space to another under the premise that the color distortion is minimal, and the appearance of the color from the input display to the output is matched as much as possible, so that the original and the product color are finally consistent. .
When the color management is implemented, the color in a device's color space is converted into the mediating color space in combination with the device's characteristic file, and then the target device's characteristic file is converted into the target device's color space. Due to the use of device-independent color spaces as mediations, color consistency is guaranteed. According to this implementation process, three elements of color management are obtained:
The device-independent color space serves as an intermediary for color conversion; for each device, there must be a standard specification describing its color characteristics and parameters; there must be a precise conversion algorithm to achieve the color conversion between different spaces.
In order to standardize the standards of color management, the International Color Consortium (ICC) has established the ICC specification, making every element of color management clear: the color space as an intermediary is CIEXYZ or CIELab; the file format describing the color characteristics of the device It is the format specified by ICC; the color matching algorithm is completed by the manufacturer according to the ICC specification.
Color management workflow
Establishing the device's color profile is the core of color management. The description file has representative color features of each device, such as chroma characteristic curve, output color gamut characteristic curve, etc. The color management system uses these color features. Implement color space matching and conversion for each device.
There are many workflows for color image reproduction based on ICC, and when the image is printed as an independent file or exists as an object in the page file, the specific implementation methods of color management are also different. According to the application time of the output device characteristic file, it can be divided into Early-binding workflow and Late-binding workflow. The former converts the color information of all the images from the source color space to the color space of the final output device as early as possible; the latter, as far as possible, delays the conversion of the color information of the image to the color space of the final output device.
The principle of color management
To perform color management, a series of prescribed operations must be followed to achieve the desired results. Color management is divided into three steps in principle, abbreviated as 3C, namely calibration (calibration), characterization (Characterization) and conversion (Conversion).
The calibration is to ensure the stability, reliability and sustainability of the color information transmission process. The input, display, and output devices must be calibrated to ensure that they are in standard operating conditions. The 3C principle should be followed for the calibration of each device.
Characterization is to record the corrected characteristics of all devices. In order for each device to achieve accurate color space conversion and matching, it must be characterized. For the input device, display and output device to make its color gamut characterization curve, based on this, make a color description file against the device-independent color space. These profiles are the bridges from device color space to standard device-independent color space (PCS). On the basis of the calibration of the system equipment, using the device description file, the standard device-independent color space is used as a medium to realize the conversion between the color spaces of the devices. Since the color gamut of the output device is narrower than the color gamut of the original document, scanner, and display, the color gamut needs to be compressed by the method in the ICC protocol during color conversion. The following describes some of the specific operations of the 3C implementation.
Input correction: Including brightness, contrast, black and white field correction. Taking the scanner calibration as an example, when the scanner is initialized to zero, the same image data should be obtained for the same original document whenever it is scanned.
The first is to adjust the relevant parameters of the scanner to the allowable range, such as the light source should not exceed the corresponding life, the reference white is clean and no color change, etc. Then the device profile of the scanner is generated, which is the core of color management. The specific customization process is to first scan a 264 color patches, represent the standard color code of the entire CIELab color space, generate an RGB file, and then use ScanOpen to read the scanned RGB files and the original standard RGB files. The color difference between these two files generates an ICC curve, which is the scanner's device profile. Finally, in the actual scan, the generated ICC file is called to complete the standard conversion of the color space.
The color space conversion of the scanner undergoes the process of generating an original RGB file from the scanner's JCC file from the original Lab color space to the scanner's RGB color space to the Lab color space. In the color management of the scanner, it should be noted that the parameters of the device will change over time. Therefore, it is necessary to regularly customize and replace ICC documents to ensure that the latest working conditions of the scanner are reflected.
Display Correction: The display is an RGB color space device that uses the display calibration system to set its brightness, contrast, color temperature, and Gamma value for the entire display system. The display characteristics of the display meet the ideal parameter values ​​set in its own device description file, so that the display card accurately reflects the color of the CMYK color space on the display screen according to the color data of the image data. Pre-press image processing should use a professional color management solution. That is, using the hardware measurement method, the device feature file of the display is generated. Such as Heidelberg's ViewOpen and matching suction cup screen colorimeter and X-Rite's PulseColorElite system. In order to ensure the accuracy of the display, the environment around the display should also be improved. If the display is equipped with a hood, reduce the interference of stray light around: install a standard light source; change the background color of the monitor desktop to neutral gray. The color display should be installed in a room where the walls and ceiling are all gray. After the above transformation, the display can meet the requirements of professional soft proofing and achieve true WYSIWYG.
Output correction: It is the last step of the calibration process. It includes corrections for printers, imagesetters, printers, and proofers. It corrects the characteristics of the equipment according to the equipment description provided by the equipment manufacturer, and makes the equipment comply with the factory-standard characteristics. Output. When doing printing and proofing corrections, it is necessary to make the printing materials such as paper and ink used in the equipment meet the standards. The gray balance and black plate parameters of the output device are corrected. This is a correction for the printing material parameters such as paper and ink. Take the digital proofing correction as an example to introduce the following.
Digital proofing is the use of color management technology to achieve the offset printing color on the inkjet printer. As the final color output link before printing, the accuracy of the color is directly related to the customer's acceptance of printed products. First of all, to calibrate and determine the equipment parameters used in the normal operation of inkjet printers, the performance of ink heads, and the types of inks and substrates normally used, ensuring that the materials used to form the device profile are the materials used in normal operations. Second, use a calibrated printer to print the standard color combination (usually the IT8.7928 color block combination). Then, standard color plates are combined and color-coded and printed on calibrated printers to find representative printed proofs. Finally, a spectrophotometer is used to measure the combination of the standard sample mark and the print proofs, and the data is input online into PrintOpen to generate the equipment profile of the inkjet printer. In use, digital proofing color management is achieved by calling digital proofing ICC profiles.
Since paper and ink used for proofing have a great influence on color, digital proofing equipment profile files should be re-customized when replacing new paper and ink. When re-customizing, it is only necessary to re-measure the newly generated standard color code combination of the inkjet sample compared with the previously input printing sample data, without reprinting.
Color Box Enterprise Color Management Implementation
System composition
A complete color management system should include input, display, and output device characteristics files, feature file creation software, and standard inspection objects needed to create the profile, color management models, profile file connection space, and application software interfaces. There are many kinds of characteristic file making software on the market at present, such as ColorFIow, ProfileEditor software and so on. The standard test object includes the objects required to acquire the scanner, display profile, and output device profile.
Can be said to do a good job in color management, the need for a standard ambient light source; high-quality color profile generation software; advanced color management system: standard measuring instruments and testing tools; standard color target: standard printing management; The remote digital proofing color management; standard scan restore color management: standard digital camera color management; standard monitor calibration and screen soft proofing.
System Implementation
In the implementation of color management, carton companies should focus on the following aspects:
Standardized management of the printing process: The standardization, standardization, and data-based production management of the various processes throughout the printing process are the prerequisites and foundations for the application of ICC color management. Enterprises should strictly establish the quality standards for fine products, standards for raw and auxiliary materials and equipment such as paper and ink, standards for measuring tools and ambient light sources, and formulate standardized management documents, and require that everyone strictly follow the standards. The ICC files and parameters of each device shall be standardized, quantified within the standards and specifications, and summed up to ensure that the quality of the data.
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Talking about the Color Management Technology of Folding Box Printing Enterprises
As the market continues to improve the quality of color box products, color management has also been more and more attention. However, in the current production, there are various color problems, high-quality manuscripts cannot be well copied, and the quality of unsuitable manuscripts does not improve after copying, making the original attractiveness of product packaging unable to perform adequately and failing to achieve the purpose of publicity and promotion. At the same time, waste products and costs are increased, which reduces the competitiveness of the company. The reason is that in addition to the problems existing in the equipment and personnel technical level, the key lies in the failure of the company to carry out effective color management.